<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>过滤器</title>
</head>
<body>
{{ letters|length }}<br>
{{ letters|upper }}<br>
{{ letters|join:',' }}<br>
{{ number|add:1 }}<br>
{{ number|add:-1 }}<br>
</body>
</html>